Driving Economically - 56mph Myth??

Adams1

@OP, it's nothing to do with the speed you go at, its got to do with the car you drive, with the type of engine that's in it... For example, a 150BHP diesel car can achieve the best MPG @ 65-70MPH due to the ammount of torque @ low RPM which also depends on the aerodynamics of the vehicle and wind speed at the time... But with the same engine, in lets say, a van... You would need to do around 50-55MPH MAX due to the sheer shape and size of the vehicle. (aerodynamics.)

dacouch

jase1 wrote: »

Ooh -- old but interesting thread.

I know that our Mitsubishi 1.6 petrol can manage 55mpg when plodding at 50mph (found this particular nugget out on one of those average speed areas) ... this seems to be this particular car's sweet spot. I remember thinking that was mightily impressive for a fairly old-school engine at the time.

56mph is far too arbitrary a number to have any real meaning. Some very small cars have their sweet spot around 40mph in my experience, whereas some very large-engined cars can be more economical at 70 than at 50 (my ex-boss's 3l Toyota Supra twin-turbo for example). Depends on the car.

56mph was probably chosen as it is 90kph, the reason they do 0 to 62mph time tests on cars is because it is in effect 0 to 100 kph

oscarward

Adams1 wrote: »

@For example, a 150BHP diesel car can achieve the best MPG @ 65-70MPH due to the ammount of torque @ low RPM which also depends on the aerodynamics of the vehicle and wind speed at the time

My 150hp diesel Jaguar X-type estate achieves 70mpg at a constant 50mph (I was running on the space saver spare wheel ) and at a steady 80mph achieved 50mpg. Both over a distance of about 35 miles on dual carriageway / motorway with a rolling start i.e. reset trip after reaching speed. 50mph in 6th gear is about 1250rpm at which revs the turbo is only just about in use.

I find my economy varies more depending on where I fill up. e.g. the 50mpg was on Asda fuel and the same journey at 67mph (ish) achieved 46 mpg on Sainsbury's diesel.

Adams1

oscarward wrote: »

My 150hp diesel Jaguar X-type estate achieves 70mpg at a constant 50mph (I was running on the space saver spare wheel ) and at a steady 80mph achieved 50mpg. Both over a distance of about 35 miles on dual carriageway / motorway with a rolling start i.e. reset trip after reaching speed. 50mph in 6th gear is about 1250rpm at which revs the turbo is only just about in use.

Not too shabby. the engine I'm reffering to is the PD150 motor. (found in the gen 4 Golf's) Gives out a good 55MPG @ 70MPH. If I'm not mistaken, the X Types + being an estate, must be a pretty heavy car... (2000kg?) The golf is around 1400kg, so you must have one solid motor, or a broken trip computer. Haha! (is it the the merc engine?)

Whats great about owning diesels are their ability to be tuned directly from the cars computer (ECU) for either better performance (150BHP > 185BHP) or better economy. I was going to get it remapped, but couldn't really part with £250 as I couldn't see a return anytime soon due to me needing a new car/van for work. But if I did, I was promised a good 100-150 miles extra out of a tank on the motorway which didn't sound to shabby. (I'd say 70-100 tops to be realistic.)

I find my economy varies more depending on where I fill up. e.g. the 50mpg was on Asda fuel and the same journey at 67mph (ish) achieved 46 mpg on Sainsbury's diesel.

This. Alot of people dive for the cheapest, or the nearest fuel station. After running my own tests for a couple of weeks (motorway runs, no cruise control, steady foot @ 65mph) I've noticed I could push a good 25+ miles extra out of a tank useing BP's fuel over Sainsbury's. Granted its not the greatest save on fuel BUT 25 miles is 25 miles. I get a free days comute.

arryzed

Driving Economically - 56mph Myth??


I remember reading about a recommendation that driving at 56mph will give the max performance efficiency in terms of driving a motor car.

It was suggested that this applied to all cars no matter what the size of engine

lets go back to the beginning this was the question as above ...if anyone read my post and understands it you will notice that I stated

Basically the magic 56mph is because the energy needed to move a car beyond this point (the drag of resistance through the air, tyres, gravity, etc) increases at such a steep curve irrespective of what vehicle you have although large heavy vehicles are a lot worse.
The graph ramp (energy/fuel to speed) starts to climb at 40mph so in real terms if all vehicles were geared to have their fuel efficiency set to 40mph we would all get even better fuel consumptions but we would have to drive at 40mph....hope this makes sense.

#travelling at speeds beyond approx 56mph the power needed to propel ANY vehicle forwards increase(irrespective of aerodynamics, although aerodynamics helps of corse it does but you can not get away from the laws of physics) some vehicles mainly diesel's but also some petrol engines can be more econmical at 40mph even 25mph, but on the whole most manufacutures design fuel efficieny at approx 56mph for a very good reason...the laws of physics please see the picture of a recent study, somewhere around 40mph would actually be the best place for ALL cars to be designed to be fuel effiecent but i dont think we would want that.

as a new user it wont let me post link
please do a google search for "Edition29_Chapter04.pdf"
and look at section 4-27 table 4.26 you will see from these tests that other than hybrid cars (which we are not talking about) there are no vehicles that have better fuel economy beyond the magic 56mph

jase1

Actually that isn't true -- look at table 4-29; the 1.8 Toyota Celica is more economical at a higher speed.

Most of the cars on that study are slab-sided, old-school US vehicles, most of which are rather old. Drag increases regardless of speed -- it's all about how a car is engineered.

American cars of that period were designed with American roads in mind -- where the speed limits are set low. Many modern European repmobiles are set up with the German autobahns in mind. 56mph is not a magic bullet, it's an arbitrary setting that is increasingly irrelevant in many areas.

Some cars are still labouring at 56mph in 6th gear -- clearly they aren't designed to be run in that manner.

arryzed

It's not that 56 is the be all and end all because its not, its actually lower down below 40mph, its at approx at 56 mph the ramp climbs at 58 mph that toyota could of had a bettre consumption that 60


56mph is not most fuel efficient point actually its somewhere just below 40mph, but we dont all want to drive around at 40mph, diesel engines are very differant than petrol engines in performace, fuel usage and power levels. and will consume less fuel the slower you drive them with low revs 100% guaranteed.

Because air resistance increases so rapidly with speed, above about 30 mph (48 km/h), it becomes a dominant limiting factor. Driving at 45 rather than 65 mph (72 rather than 105 km/h) requires about one-third the power to overcome wind resistance, or about one-half the energy per unit distance, and much greater fuel economy can be achieved. Increasing speed to 90 mph (145 km/h) from 65 mph (105 km/h) increases the power requirement by 2.6 times, the energy per unit distance by 1.9 times, and decreases fuel economy.

[FONT=&quot]The graph ramp (energy/fuel to speed) starts to climb at 40mph and rise's a little more to approx 56mph, [/FONT]but [FONT=&quot]travelling at speeds beyond approx 56mph the graph raises dramaticaly the power needed to propel ANY vehicle forwards increase (irrespective of engine or aerodynamics, although aerodynamics helps of corse it does, but you can not get away from the laws of physics) some vehicles mainly diesel's but also some petrol engines can be more econmical at 40mph even 25mph, but on the whole most manufacutures of PETROL engines design fuel efficieny at approx 56mph for a very good reason[/FONT].....I should of mentioned that diesels are differant in that they can have a greatly improved fuel efficienty at slower speeds than petrol engines, petrol engines can become in many case's worse at slower speeds, but ALL vehicles as stated above Diesel or petrol will use more fuel at 65mph compared to 56 mph ....because of the laws of physics

Mark_Hewitt

Quite often manufacturers figures for motorway driving performance are at something like 60mph, so they are geared to return maximum MPG at that figure.

oscarward

Adams1 wrote: »

Not too shabby. the engine I'm reffering to is the PD150 motor. (found in the gen 4 Golf's) Gives out a good 55MPG @ 70MPH. If I'm not mistaken, the X Types + being an estate, must be a pretty heavy car... (2000kg?) The golf is around 1400kg, so you must have one solid motor, or a broken trip computer. Haha! (is it the the merc engine?)

Actually it's the 2.2l 4 cylinder engine as fitted to Mondeo, Peugeot 407 and Citroen C5 with a 6 speed gearbox. In PSA form it outputs 170hp and reading the motoring press the latest Mondeo / Peugeot 508 2.2d now produces 204hp. Not too bad as my previous Pug 407 V6 2.7d produced 205hp.

The car weighs 1575kg which gives a superb ride. Must try and see what I get at 70mph when I go up to Keele tomorrow, just out of curiosity as I'm on Morrisons fuel at the moment. 5p off a litre if you spend £40 in store.:beer:

jonny19719

I haven't read all seven pages on this thread so someone may have answered this, but there is a simple answer to the original question. Putting aside all of the maths calulations and comparisons between cars the answer is:-

The lowest engine revs in top gear without labouring the engine.

This means every car will be different, my Audi A3 diesel with 6 gears will acheive the best MPG at 47mph, at this speed the engine revs are about 1500 so not so slow to strain the engine, yet it's the greatest speed for the lowest revs.

John